With the recent trend toward the digitization of medical X-ray images, X-ray intensity spatial distributions can be acquired as digital images. For example, currently available schemes include a scheme of forming a latent image on a photostimulable phosphor by using X-ray energy and acquiring an image from a laser pumping light distribution, a scheme of converting an X-ray intensity distribution into a light intensity distribution (fluorescence), directly converting the distribution into an electrical signal by using a surface center having a plurality of pixels, and converting the signal into a digital image, and a scheme of directly converting an X-ray intensity distribution into a charge distribution.
The digitization of X-ray images has the following merits:                Storage and transfer can be efficiently performed.        Optimal images can be easily formed by digital image processing (a recovery from a failure in imaging operation can be made).        Efficient diagnosis can be carried out.        A reduction in the cost of diagnosis can be attained.        
The problem of scattered X-rays produced when X-rays pass through an object has not been satisfactorily solved. To reduce the influences of scattered X-rays and obtain a high-contrast image, an optimal means is to use a scattered ray removing grid having many lead plates arranged in the same direction as in a conventional scheme using silver-halide films.
FIG. 10 is a schematic sectional view of a structure using a grid. Reference numeral 81 denotes a point (X-ray focal point) of an X-ray tube from which X-rays are generated; 82, an object; 83, a grid; 84, an energy conversion unit for converting an X-ray intensity distribution into a light intensity or charge amount; and 85, a sensor unit for spatially sampling the distribution. Scattered X-rays reflected by the object reach the grid 83 as well as X-rays directly emitted from the X-ray tube. Most of the scattered X-rays are cut by the grid 83 made up of lead members facing the X-ray focal point 81.
A drawback of the grid 83 is that it partly cuts direct X-rays while it cuts scattered X-rays. This cutting pattern corresponds to the arrangement of the lead members of the grid, and the image generally suffers a stripe pattern. An X-ray image has evolved into (1) an image formed by a film-screen system (analog image)→(2) a digital image formed by reading a latent image formed by a photostimulable phosphor by laser scanning→(3) an image formed by direct sampling (flat panel sensor) the two-dimensional spatial distribution of an X-ray dose in a two-dimensional space. Different measures have therefore been taken against a stripe pattern (grid image) on an image which originates from the lead members used for the grid 83.
In the film-screen system in (1), the following two methods are available, which are used to remove a grid image or prevent interference with observation.
(a) The grid itself is moved during radiation of X-rays to prevent the formation of a grid image while removing scattered rays.
(b) The spatial frequency of the grid stripe pattern is increased to make it difficult for the human eye to perceive a grid image if it is formed on an image or prevent the grid image from overlapping the frequency component of image information.
The means of moving the grid itself in (a) is effective in all cases of X-ray image acquisition. However, this means is difficult to use because of an increase in cost due to, for example, a driving system for moving the grid, an increase in apparatus size, the relationship between the driving timing and the X-ray radiation timing, adjustment of the driving speed, and the like.
The means of increasing the spatial frequency of the grid stripe pattern in (b) has its own limit. That is, when the frequency of the grid stripe pattern is set to a high spatial frequency at which no grid image is formed, since the thickness of each lead plate for blocking scattered rays is almost fixed, an area through which direct rays are transmitted narrows, and the use efficiency of the X-ray dose extremely decreases. As a consequence, imaging operation cannot be properly performed.
In the era in which a latent image formed by a photostimulable phosphor was read by laser scanning and digitized, the idea of using anti-aliasing filter before sampling was introduced as a method of removing a grid image. When a latent image formed by a photostimulable phosphor is to be read by laser scanning and digitized, the image is scanned in a one-dimensional direction by a laser to temporarily form a signal form like a video signal, and the signal is sampled on the time axis. The frequency of the grid stripe pattern is increased to a certain degree, and laser scanning is performed in a direction perpendicular to the grid stripe pattern to form the grid stripe pattern into a periodic signal on the video signal. A grid image can be removed by the general idea of using an anti-aliasing filter, i.e., performing sampling on the time axis after low-pass filtering in the state of an analog signal as this video signal. A similar method is disclosed in Japanese Patent No. 2507659, in which a grid image and its frequency are obtained by Fourier transformation of an image obtained by preliminary sampling, and a low-pass filter corresponding to the result is selected to remove a grid image.
According to another method, sampling is performed on the time axis at intervals shorter than desired intervals instead of performing analog low-pass filtering to eliminate aliasing of grid stripe pattern information, and the resultant information is separated from image information. Thereafter, digital low-pass filtering is performed, and the resultant image is digitally decimated (sub-sampled), thereby obtaining an image at the desired sampling intervals. Similar methods are disclosed in Japanese Patent No. 2754068 and Japanese Patent Laid-Open No. 8-088765.
In the advanced era in which a digital X-ray image can be obtained by directly sampling (using a flat panel sensor) a two-dimensional spatial distribution of an X-ray dose in (3) in a two-dimensional space, the above anti-aliasing filtering cannot be used. That is, a flat panel sensor is made up of a plurality of semiconductor pixels, and the two-dimensional spatial sampling pitch of the sensor cannot be reduced more than necessary in consideration of technique and cost. The above idea of using an anti-aliasing filter cannot be applied to this method. The method disclosed in Japanese Patent Laid-Open No. 9-75332 is aimed at removing grid stripe pattern information in contrast to the method of obtaining an X-ray image by direct sampling in a two-dimensional space. In this method, the intervals of grid lead members are perfectly matched with the sampling pitch to match areas where direct X-rays are blocked by the grid stripe pattern with the gaps between the pixels, thereby preventing a grid stripe pattern from appearing on an image.
Japanese Patent Laid-Open No. 9-98970 and U.S. Pat. No. 5,801,385 disclose methods of setting the grid lead member intervals to be smaller than the sampling pitch and equal to or near the width of the opening of a light-receiving portion of one pixel, thereby reducing the contrast of a grid stripe pattern. In the method disclosed in U.S. Pat. No. 5,050,198, a grid image is input and stored under a plurality of conditions. When imaging operation is actually performed by using the grid, the obtained image is divided by a grid image of the stored grid images which corresponds to the condition under which the actual imaging operation is performed, thereby removing the grid image.
In the method disclosed in Japanese Patent Laid-Open No. 9-75332, which corresponds to the above technique of obtaining a digital X-ray image by direct sampling in a two-dimensional space using a flat panel in the two-dimensional space, it is very difficult to perfectly match the grid lead member intervals with the sampling pitch. In the methods disclosed in Japanese Patent Laid-Open No. 9-98970 and U.S. Pat. No. 5,801,385, a grid image can be effectively removed by reducing the grid lead member intervals below the sampling pitch to be equal or near the width of the opening of a light-receiving portion of one pixel. However, as the flat panel sensor increases in resolution, and the sampling pitch becomes 0.1 mm or less, the grid lead member intervals are required to be very small; 10 or more grid lead members per mm. If the intervals become so small, since the thickness of each lead plate for blocking scattered rays is almost fixed, the areas through which direct rays pass narrow, and the use efficiency of the X-ray dose becomes extremely low. As a consequence, proper imaging operation cannot be performed.